The Visual System:Retinal Mechanisms

Lesson 16

The light stimulus

Electromagnetic energy visible light 350 – 700 nm (violet – red)

Photon Wave

Color (hue) - wavelength Brightness (intensity) – amplitude ~

Amplitude

Wavelength

Brightness

Color

Retina

LVF RVF

Optic Nerve

Optic Chiasm

Optic Tract

LGN LGNLateral GeniculateNucleus (Thalamus)

Optic Radiations

V1 V1Primary Visual Cortex

Visual Pathway

Retinal Organization Photoreceptors (PR)

transduction Bipolar cells (BP)

no APs Retinal ganglion cells (RGC)

1st APs in pathway Amacrine & Horizontal cells

communication b/n retinal cells parallel to retina ~

Retinal Organization

PR

BP

RGC

Which directiondoes the lightenter?

Light

Optic Nerve

Photoreceptors (PR)

Transduction Rods

achromatic Cones

color ~

Photoreceptors - PR Dark current

Depolarized in dark Na+ influx NT is released

BP inhibited ~

Photoreceptors - PR Light strikes PR

Receptor potential hyperpolarization NT release

BP depolarizes NT release

Excites RGC APs ~

Rods

Around edges of retina 120 million Pigment = rhodopsin

Convergence high sensitivity low acuity ~

Cones

Color vision - 3 types red = long wavelength green = medium blue = short

6 million Concentrated in fovea

Little convergence low sensitivity high acuity ~

Receptive Field (RF)

Region of the retina where... changes in illumination... will change the activity of a particular neuron

Center-Surround Organization BP RGC LGN neurons convergence ~

Center-Surround Organization

PR BP RGC

Center-Surround Organization

OFF

ON

ON

OFF

2 arrangements

Center-Surround Organization

Light strikes On area of RF RGC excited APs

Light strikes Off RGC inhibited APs

Contrast maximal effects on RGC center & surround lighted cancels

out ~

Lateral Inhibition

Center-surround RF Horizontal cells

no APs PR stimulates HC inhibit adjacent BPs

Enhances contrast ~ G

PR

-

BP

+G

BP

+

PR

-H

-

+

Dark Light

Surround Center